The sleep apnea syndrome afflicts an estimated 1% to 5% of the general population and is due to episodic upper airway obstruction during sleep. Those afflicted with sleep apnea experience sleep fragmentation and intermittent, complete or nearly complete cessation of ventilation during sleep with potentially severe degrees of oxyhemoglobin desaturation. These features may be translated clinically into extreme daytime sleepiness, cardiac arrhythmias, pulmonary-artery hypertension, congestive heart failure and/or cognitive dysfunction. Other sequelae of sleep apnea include right ventricular dysfunction with cor pulmonale, carbon dioxide retention during wakefulness as well as during sleep, and continuous reduced arterial oxygen tension. Hypersomnolent sleep apnea patients may be at risk for excessive mortality from these factors as well as by an elevated risk for accidents while driving and/or operating potentially dangerous equipment.
Although details of the pathogenesis of upper airway obstruction in sleep apnea patients have not been fully defined, it is generally accepted that the mechanism includes either anatomic or functional abnormalities of the upper airway which result in increased air flow resistance. Such abnormalities may include narrowing of the upper airway due to suction forces evolved during inspiration, the effect of gravity pulling the tongue back to appose the pharyngeal wall, and/or insufficient muscle tone in the upper airway dilator muscles. It has also been hypothesized that a mechanism responsible for the known association between obesity and sleep apnea is excessive soft tissue in the anterior and lateral neck which applies sufficient pressure on internal structures to narrow the airway.
The treatment of sleep apnea has included such surgical interventions as uvulopalatopharyngoplasty, gastric surgery for obesity, and maxillo-facial reconstruction. Another mode of surgical intervention used in the treatment of sleep apnea is tracheostomy. These treatments constitute major undertakings with considerable risk of postoperative morbidity if not mortality. Pharmacologic therapy has in general been disappointing, especially in patients with more than mild sleep apnea. In addition, side effects from the pharmacologic agents that have been used are frequent. Thus, medical practitioners continue to seek non-invasive modes of treatment for sleep apnea with high success rates and high patient compliance including, for example in cases relating to obesity, weight loss through a regimen of exercise and regulated diet.
Recent work in the treatment of sleep apnea has included the use of continuous positive airway pressure (CPAP) to maintain the airway of the patient in a continuously open state during sleep. For example, U.S. Pat. No. 4,655,213 and Australian patent AU-B-83901/82 both disclose sleep apnea treatments based on continuous positive airway pressure applied within the airway of the patient.
Also of interest is U.S. Pat. No. 4,773,411 which discloses a method and apparatus for ventilatory treatment characterized as airway pressure release ventilation and which provides a substantially constant elevated airway pressure with periodic short term reductions of the elevated airway pressure to a pressure magnitude no less than ambient atmospheric pressure.
U.S. Pat. No. 5,199,424 and published PCT Application No. WO 88/10108 describes a CPAP apparatus which includes a feedback system for controlling the output pressure of a variable pressure air source whereby output pressure from the air source is increased in response to detection of sound indicative of snoring. According to additional embodiments of the apparatus disclosed in these references, a pressure ramp cycle (i.e., a gradual increase in output pressure) may occur upon initial activation of the apparatus which gradually increases output pressure from a predetermined minimum to a predetermined maximum or therapeutic pressure specifically selected for the patient.
Publications pertaining to the application of CPAP in treatment of sleep apnea include the following:    1. Lindsay, D A, Issa F G, and Sullivan C. E. “Mechanisms of Sleep Desaturation in Chronic Airflow Limitation Studied with Nasal Continuous Positive Airway Pressure (CPAP), “Am Rev Respir Dis, 1982; 125: p. 112.    2. Sanders N H, Moore S E, Eveslage J. “CPAP via nasal mask: A treatment for occlusive sleep apnea, Chest, 1983; 83: pp. 144-145.    3. Sullivan C E, Berthon-Jones M. Issa F G. “Remission severe obesity-hypoventilation syndrome after short-term treatment during sleep with continuous positive airway pressure, Am Rev Respir Dis, 1983; 128: pp. 177-181.    4. Sullivan C E, Issa F G, Berthon-Jones M., Eveslage J. “Reversal of obstructive sleep apnea by continuous positive airway pressure applied through the nares, Lancet, 1981; 1: pp. 862-865.    5. Sullivan C E, Berthon-Jones M. Issa F G. “Treatment of obstructive apnea with continuous positive airway pressure applied through the nose. Am Rev Respir Dis, 1982; 125: p. 107. Annual Meeting Abstracts.    6. Rapoport D M, Sorkin B, Garay S M, Goldring R N. “Reversal of the ‘Pickwickian Syndrome’ by long-term use of nocturnal nasal-airway pressure,” N Engl J. Med, 1982; 307: pp. 931-933.    7. Sanders M H, Holzer B C, Pennock B E. “The effect of nasal CPAP on various sleep apnea patterns, Chest, 1983; 84: p. 336. Presented at the Annual Meeting of the American College of Chest Physicians, Chicago Ill., October 1983.    8. Sanders, M H. “Nasal CPAP Effect on Patterns of Sleep Apnea”, Chest, 1984; 86: 839-844.
Although mono-level positive airway pressure or CPAP has been found to be very effective and well accepted, it suffers from some of the same limitations, although to a lesser degree, as do the surgery options; specifically a significant proportion of sleep apnea patients do not tolerate CPAP well. Thus, development of other viable non-invasive therapies has been a continuing objective in the art.